I. Related Applications
The present application is related to a co-pending U.S. patent application entitled xe2x80x9cPILOT SIGNAL DETECTION FILTER FOR A WIRELESS COMMUNICATION DEVICExe2x80x9d, filed on even date herewith, and assigned to the assignee of the present invention.
II. Field of the Invention
The present invention relates to wireless communication systems. More particularly, the present invention relates to a novel and improved method and apparatus for dynamically varying the response of a digital filter in a pilot signal detection circuit in response to the number of pilots that are scheduled to be searched.
III. Description of the Related Art
In the field of wireless communications, several technology-based standards exist for controlling communications between a mobile station, such as a cellular telephone, Personal Communication System (PCS) handset, or other remote subscriber communication device, and a wireless base station. These include both digital-based and analog-based standards. For example, among the digital-based cellular standards are the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95 series including IS-95A and IS-95B, entitled xe2x80x9cMobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System.xe2x80x9d Similarly, among the digital-based PCS standards are the American National Standards Institute (ANSI) J-STD-008 series, entitled xe2x80x9cPersonal Stationxe2x80x94Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems.xe2x80x9d Other non-CDMA based digital standards include the time-division multiple access (TDMA) based Global System for Mobile Communications (GSM), and the U.S. TDMA standard TIA/EIA IS-54 series.
The spread spectrum modulation technique of CDMA has significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
Space or path diversity is obtained by providing multiple signal paths through simultaneous links from a mobile user through two or more cell-sites. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing a signal arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31, 1992, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both assigned to the assignee of the present invention and incorporated by reference herein.
The deleterious effects of fading can be further controlled to a certain extent in a CDMA system by controlling transmitter power. A system for cell-site and mobile unit power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, Ser. No. 07/433,031, filed Nov. 7, 1989, also assigned to the assignee of the present invention. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
The aforementioned patents all describe the use of a pilot signal used for acquisition in a CDMA wireless communication system. At various times when a wireless communication device such as a cellular or PCS telephone is energized, it undertakes an acquisition procedure which includes, among other things, searching for and acquiring the pilot channel signal from a base station in the wireless communication system. For example, demodulation and acquisition of a pilot channel in a CDMA system is described in more detail in copending U.S. patent application Ser. No. 08/509,721, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference. When more than one pilot channel can be acquired by the wireless communication device, it selects the pilot channel with the strongest signal. Upon acquisition of the pilot channel, the wireless communication device is rendered capable of acquiring additional channels from the base station that are required for communication. The structure and function of these other channels is described in more detail in the above referenced U.S. Pat. No. 5,103,459 and will not be discussed in detail herein.
The base station pilot signals are divided into four sets based on their pilot signal strengths as measured at the mobile station. The first set, referred to as the Active Set, comprises base stations which are currently in communication with the mobile station. The second set, referred to as the Candidate Set, comprises base stations which have been determined to be of sufficient strength to be of use to the mobile station. Base stations are added to the candidate set when their measured pilot energy exceeds a predetermined, threshold TADD. The third set is Neighbor Set which is the set of base stations which are in the vicinity of the mobile station (and which are not included in the Active Set or the Candidate Set). And the fourth set is the Remaining Set which consists of all other base stations.
In an IS95-A communication system, the mobile station sends a Pilot Strength Measurement Message when it finds a pilot of sufficient strength that is not associated with any the of the Forward Traffic Channels currently being demodulated or when the strength of a pilot that is associated with one of the Forward Traffic Channels being demodulated drops below a threshold for a predetermined period of time. The mobile station sends a Pilot Strength Measurement Message following the detection of a change in the strength of a pilot under the following three conditions:
1. The strength of a Neighbor Set or Remaining Set pilot is found above the threshold TADD.
2. The strength of a Candidate Set pilot exceeds the strength of an Active Set pilot by more that a threshold (TCOMP).
3. The strength of a pilot in the Active Set of Candidate Set has fallen below a threshold (TDROP) for greater than a predetermined time period.
The Pilot Strength Measurement Message identifies the base station and the measured pilot energy in decibels.
Typically, a conventional mobile station includes only a single search engine which it must use to serially search for all pilots of interest. The mobile station will typically search all of the set populations according to a predetermined schedule, spending a predetermined amount of time searching for each pilot signal. Thus, no matter how many pilots need to be searched, the single search engine must search each of them serially. It can be readily appreciated that in a complex communication system, such as an urban area where there are many base stations, the total time required to search all of the individual pilots serially may increase to an unacceptably high value. A long list of neighbor pilots to search generally corresponds to a slow response time to promote any particular pilot.
Thus, the mobile station""s search engine may be so burdened by the number of pilots that it must search that it is late in detecting a relatively strong pilot that appears. In a CDMA communication system, a neighboring base station""s pilot signal causes interference to the mobile station unless and until it is successfully demodulated and promoted to the active set. The large number of pilots being searched by one search engine caused the mobile station to be late in reacting to a changing communication environment.
For these reasons, there is a clear felt need for a method and apparatus for dynamically varying the response of a digital filter in a pilot signal detection circuit which is not susceptible to these excessive delays.
The present invention is a novel and improved method and apparatus for dynamically varying the response of a digital filter in a pilot signal detection circuit in response to the number of pilots that are scheduled to be searched. The apparatus comprises a searcher subsystem for generating a pilot energy sample. The pilot energy sample is then digitally filtered in a filter whose coefficients are determined in response to the number of pilots that are scheduled to be searched. A control processor determines the number of pilot signals to be searched, and calculates a filter length in response to the number of pilot signals to be searched. Furthermore, the control processor filters the pilot energy sample according to filter coefficients which are based on the calculated filter length.
In the preferred embodiment, the calculated filter length is inversely proportional to the number of pilot signals to be searched, thereby reducing the amount of time spent on each individual pilot signal. For the same reason, in the preferred embodiment, the calculated filter length is inversely proportional to a search window size used to search the pilot signals.
Once the pilot energy samples are filtered, the result is compared to a predetermined threshold. A message generator generates a pilot strength measurement message in response to the comparison. A receiver receives a response message in response to the pilot strength measurement message, and the control processor recalculates the filter length in response to the response message.